Investing in mutual funds for retirement is never too late. And the Zacks Mutual Fund Rank can be an excellent tool for investors looking to invest in the best funds.

How can you tell a good mutual fund from a bad one? It's pretty basic: if the fund is diversified, has low fees, and shows strong performance, it's a keeper. Of course, there's a wide range, but using the Zacks Mutual Fund Rank, we've found three mutual funds that would be great additions to any long-term retirement investors' portfolios.

Here are the funds that have achieved the Zacks Mutual Fund Rank #1 (Strong Buy) and have low fees.

Fidelity Growth Strategies Fund K

(FAGKX): 0.66% expense ratio and 0.56% management fee. FAGKX is an All Cap Growth mutual fund investing in a wide variety of equities, no matter the size of the company and as long as the firm exhibits growth characteristics. With annual returns of 11.4% over the last five years, this fund is a winner.

GMO U.S. Equity Allocation Fund ClIII

(GMUEX). Expense ratio: 0.48%. Management fee: 0.46%. GMUEX is a Large Cap Growth option; these mutual funds purchase stakes in numerous large U.S. companies that are expected to develop and grow at a faster rate than other large-cap stocks. This fund has managed to produce a robust 15.73% over the last five years.

Principal Mid Cap R4

(PMBSX) is an attractive large-cap allocation. PMBSX is a Mid Cap Growth mutual fund. Mid Cap Growth funds pick stocks--usually companies with a market cap between $2 billion and $10 billion--that demonstrate extensive growth opportunities for investors compared to their peers. PMBSX has an expense ratio of 0.96%, management fee of 0.58%, and annual returns of 12.06% over the past five years.

We hope that your investment advisor (if you use one) has you invested in one or all of the top-ranked mutual funds we've reviewed. But if that isn't the case, it might be time to have a conversation or reconsider this vitally important relationship.

Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report

Get Your Free (FAGKX): Fund Analysis Report

Get Your Free (GMUEX): Fund Analysis Report

Get Your Free (PMBSX): Fund Analysis Report

To read this article on Zacks.com click here.

Zacks Investment Research